Non-invasive measurement of the concentration of a compound or analyte in a part of a subject, such as the finger, arm or earlobe, may be difficult in cases where there is interfering background absorption of the same, or other analytes, within the body part from non-target compartments. Changes in the volume of the different compartments may also adversely impact on readings obtained for the determination of a compound within a part of the body.
Compartments within tissue of the body may include, but are not limited to the vascular, interstitial, cellular, lymph, connective tissue, and bone compartments. In the case where the interfering background absorption is from the same analyte present in a non-target compartment, then the concentration of the analyte in the compartment of interest may be overestimated. Changes in the volume of different compartments within the light path, may effect the determination of the concentration of the compound. In order to determine the total amount of a compound within a body part, the occurrence of the compound within each of the compartments may be required. This may be important in cases where the relative amount of a compound of interest may vary within different compartments over time or as a result of a medical condition.
For example, when the method disclosed in U.S. Pat. No. 5,361,758 (Hall et al.) is used to measure the blood glucose concentration in diabetic patients adhering to an insulin regime, the background glucose concentration in the cells and the interstitial fluid is negligible and does not interfere significantly with the measured plasma glucose concentration. However, any excess glucose in the blood is eliminated through urination, resulting in dehydration of the patient as water is continuously removed from the tissues of the body and from the interstitial fluid. As a result of the decrease in the cellular and interstitial fluid volumes, the effective glucose concentrations in the cellular and interstitial compartments increases. The increase of glucose in non-target compartments can interfere with the measurement of the blood glucose concentration when using non-invasive measurement techniques, such as that disclosed in Hall et al. This overestimation can result in an inaccurate reading of blood glucose levels. The development of a process that is able to determine the concentration of a particular analyte in different compartments of a part of an individual is therefore of importance.
Clinical studies have revealed that the concentration of certain compounds in one particular compartment of a part of a subject, such as the skin, may be used to assess the risk of development of specific medical conditions in that subject. Early detection of these types of risks in a patient permits measures to be taken that may slow or even prevent the onset of these conditions. As an example, it has been determined that elevated concentrations of cholesterol in the skin of an individual is an indication of a risk for coronary disease. Therefore, the development of simple, non-invasive methods for determining the concentration of skin compounds is of importance.
In U.S. Pat. No. 6,365,363, Parfenov et al. describe a method of indirectly measuring the concentration of cholesterol in the skin of a subject by enzymatically oxidizing the cholesterol in a section of the subject's skin and then quantitating the amount of the hydrogen peroxide by-product stoichiometrically formed in this reaction using a second enzymatic reaction. As a complex series of enzymatic reactions are used in this method to indirectly determine the concentration of cholesterol, the method is both costly and prone to error. In addition, the development of a result using this method is time consuming.
In U.S. Pat. Nos. 6,236,047 and 6,040,578, Malin et al. describe a method for determining the concentration of a blood compound using light in the nearinfrared range by analysing diffusively reflecting radiation emerging from the irradiated sample. However, there is no teaching in these patents as to the determination of concentrations of constituents in the various compartments of a part of a subject.
Hall et al. also describe in U.S. Pat. No. 5,361,758 a non-invasive technique for directly measuring the concentration of constituents of blood using light in the near-infrared range. The glucose value is referenced with respect to the blood compartment only and the glucose concentrations obtained using this method may be prone to error arising from changes in the fluid content in other compartments.
It is an object of the invention to overcome disadvantages of the prior art.
The above object is met by the combinations of features of the main claims, the sub-claims disclose further advantageous embodiments of the invention.